Multiple antenna element active null steering antennas are particularly useful in environments in which interfering signals, such as intentionally or unintentionally applied jamming signals, may otherwise adversely affect position determination based on GNSS satellite signals. The signals received by the respective antenna elements are weighted and combined to effectively produce one or more nulls that cancel out the jamming signals. As appropriate, the weighting also steers the antenna beam toward, or increases the gain of the antenna in the direction of, one or more transmitters. The active null steering antenna of interest is a controlled radiation pattern antenna (CRPA).
The signals received by the antenna elements of the CRPA are analyzed to determine the direction and elevation of a jamming signal, if present. The signals from the respective antenna elements are then weighted to provide a null that essentially cancels out the jamming signal, and thus, eliminates the contributions of the jamming signal to the signals that are processed for position determination. In addition, the weighting applied to the signals may also steer the beam toward the GNSS satellites in view, based on a single overhead pointing constraint, such as pointing skyward, that increases the gain in the desired direction. Multiple pointing constraints may be used to direct the antenna beam toward particular satellites or groups of satellites and/or to avoid multipath signals, and so forth. Alternatively, the constraints may be utilized to steer the antenna beam toward a given satellite and nulls may be produced around the beam, to effectively eliminate the contributions of both multipath and jamming signals to the signals that are analyzed for position determination.
The CRPAs work well to provide satellite signals from which pseudoranges can be determined based on code delays in an environment in which jamming signals are present. However, precision GNSS position determinations are made using carrier phase measurements of the respective GNSS satellite signals. Typically, short baseline real time kinematic (RTK) information is utilized by the receiver, or rover, operating with the CRPA, allowing the rover to quickly resolve carrier cycle ambiguities, correct for atmospheric conditions and satellite position errors, and so forth, such that positions with centimeter accuracies can then be determined from the carrier phase measurements of a plurality of GNSS satellite signals.
The systems employing short baseline RTK information typically operate with a base GNSS receiver and a rover GNSS receiver that are separated by a small number of kilometers, for example, less than 10 kilometers. The base receiver, which is in a known position, makes carrier phase measurements using GNSS signals transmitted from GNSS satellites in view and calculates pseudoranges from the respective satellites. The base receiver then determines differences between the pseudoranges calculated using the satellite signals and the ranges based on the known position of the receiver and the known locations of the satellites, to determine pseudorange correction information. The base receiver operating in a known manner broadcasts the RTK information, that is, the range correction information, pseudoranges, carrier phase measurements and various other information, to the rover receiver.
The rover receiver utilizes the broadcast pseudoranges, carrier phase measurements and other information to resolve integer carrier cycle ambiguities using well known, processing intensive, operations. The rover receiver utilizes the range correction information to correct for pseudorange errors related to changes in satellite orbits, atmospheric conditions, and so forth, that affect both the base receiver and the rover receiver in the same manner due to the short baseline between the receivers, all in a known manner.
Certain short baseline systems utilize fixed baselines that may, for example, employ two antennas situated at the rover receiver to determine the orientation or azimuth of the rover receiver. The antennas may be fixed to a vehicle, such as an automobile or a ship, and may, for example, be spaced apart by as little as 1 to ½ meter. Each antenna provides information that is utilized in well known short baseline RTK processing-intensive operations, to simplify the calculations involved in resolving the integer carrier cycle ambiguities for the respective antennas. Once integer carrier cycle ambiguities are resolved, the system can determine the azimuth or orientation of the vehicle based on the differences in the carrier phases measured at the two antennas. As is well known in the art, the short fixed baseline essentially reduces the complexity of resolving integer cycle ambiguities by changing the solutions from those of a 3-dimensional problem to those of a 2-dimensional problem.
Unfortunately, the active null and beam formation steering of the CRPA does not preserve the carrier phases of the received GNSS satellite signals. Instead, the active null and beam steering, that is, the application of the associated weightings to the signals received by the respective antenna elements, has the effect of moving the antenna phase center to different locations for the respective satellite signals. Accordingly, the null and beam steering introduces into the carrier phase measurements errors that adversely affect position determination since the carrier phase measurements from multiple satellites are required. Further, the short base line RTK information cannot be used effectively or at all to resolve carrier cycle ambiguities since the antenna phase center movements effectively result in different endpoints for the carrier phase measurements for the respective satellites, and thus, the short base lines can no longer be considered fixed.
What is needed is a mechanism to determine and correct for the movements of the antenna phase center attributable to the weighting of the signals from the plurality of antenna elements in an active null steering CRPA. Further, what is needed is such a mechanism that works with existing GNSS receivers that operate with CRPAs such than RTK information can be effectively utilized.